It is known from Swedish patent specification No. 40869 to prepare fuel briquettes from comminuted wood refuse (small branches, twigs, roots etcetera) while admixing 5-15% charcoal as gas binding agent; the charcoal does not appear to act as a binder for the briquettes since it must be assumed that its gas binding effect is due to the known adsorptive effect of activated carbon.
From British patent specification No. 1,286,532 it is known to produce fuel briquettes from refuse or other waste material by removing metals and fines, crushing and drying the waste or refuse and mixing it with pulverized coal (pitcoal) or coke to enrich the calorific value, and then before briquetting at a temperature less than 225.degree. C. to add a bonding agent. A simplification of this method is stated in Danish patent specification No. 143,859 to consist in mixing those parts of the refuse or waste that are to be briquetted with coal dust after which the mixture is briquetted. It is stated that hereby one avoids both to heat the refuse or waste before the pressing, and to add a bonding agent. It is not clear whether the coal dust acts as a bonding agent, or whether the material in itself has properties such that a bonding agent is not needed.
British patent specification No. 2,112,809A proposes to recover fuel from coal ash by adding water and a binder to said ash, agitating the mixture to allow coarse particles of high coal content to be formed, separating fine particles of high ash content from said coarse particles and dehydrating the coarse particles so as to be able to utilize them as a fuel. The specification does not explain how to use said coarse particles as a fuel but it seems fair to assume that the intention is to use them as such in coal dust fired furnaces.
International Patent Publication WO No. 83/04049 relates to a method of manufacturing combustible pills or briquettes of straw or other combustible material and the method is characterized in that fly ash is used as a binding agent and that the substantially dry material is compacted at a pressure sufficient to generate in the material a temperature of at least 75.degree. C., preferably 100.degree.-200.degree. C. It is stated in a sub-claim that the compaction pressure is preferably 250-2500 kp/cm.sup.2 of the material, and according to another sub-claim there is preferably used a fly ash which is "rich in residual carbon", such fly ash being employed in an amount above 10%.
It is not defined more fully what is meant with the expression "rich in residual carbon" but in the specification it is mentioned that good (attractive) fuel briquettes may be prepared by compaction of pure fly ash of a type holding some 80-85% of coal (carbon). Elsewhere in the specification it is said that there are some types of fly ash with up to 80-90% carbon. This piece of information is hardly correct, or is only correct in so far as exceptional cases are concerned. According to type of fuel (oil, coal, lignite) and type of combustion plant the content of carbon in fly ash varies from about 10% to about 50%, in rare cases perhaps up to around 70%.
It seems to appear from the WO publication that the higher the content of carbon in the fly ash, the higher the content thereof in the fuel briquettes must be. This firstly appears from the requirement as to an amount of at least 10% fly ash as bonding agent when it is a high percentage fly ash, and secondly from the statement that a fly ash containing only 10% carbon is perfectly usable in straw briquettes when the latter contain only 5% fly ash. From this one can deduce that it is the ash components of the fly ash that act as a binding agent, both in straw briquettes and in "pure" fly ash briquettes.
It has been found, however, that there is a definite disadvantage in using fly ash as a binder in biomass and industrial waste briquettes. This disadvantage is that the ash components of the fly ash cause a strong wear on the presses used for the manufacture of the briquettes. It is fair to assume that this wear is the less the more carbon there is in the fly ash, but since the amount of carbon therein as mentioned very seldom and exceptionwise is above 50%, and since for obvious reasons one makes heavy efforts to conduct the combustion of the fuel, notably in larger plants such as power stations and big district heating stations, in a manner so as to minimize the carbon content of the fly ash, the availability of such fly ash types having a very high content of carbon may be expected to decrease rather than increase.
Fly ash consists of discrete particles having very variable particle size, in fly ash from coal dust fired plants mainly of 3-300 .mu.m, from roast furnace plants (stoker plants) of 5-500 .mu.m. The particles are of two types, ash particles which are substantially spherical, frequently hollow, and consist quite predominantly of mineral material; and carbon particles which have irregular shape and mainly consist of carbon. However, mixed particles may occur wherein mineral material and carbon are present in more uniform amounts.
It is known that by flotation one can separate the fly ash into two fractions of which one quite predominantly contains mineral components and the other predominantly coal (carbon). Such a separation process which is particularly advantageous is described in U.S. Pat. No. 4,426,282 and its counterparts in other countries such as GB No. 2,092,918 or DE No. 3,205,385. It consists in flotating fly ash in at least two steps, pH being adjusted in the first step at 6-8 and in the second step at at least one pH unit lower, preferably at pH 3-5. By optimizing the process parameters (collector, frother, temperature, aeration, time for adding chemicals) it is possible to obtain a carbon fraction. In the present specification with appended claims this carbon-rich fraction is called fly ash coke and it has a content of up to 85% or 90% of carbon or even more.
Fly ash coke cannot be named fly ash. For, it is found that the abovementioned mineral particles are absent or only present in a small amount, at most 5% by weight and normally considerably less. Mineral material accordingly is substantially only present as impurities in the irregularly shaped coke particles (coal particles, carbon particles). The amount thereof first and foremost depends upon the source of the fly ash, i.e. the quality of the fuel the combustion of which has caused the fly ash formation. In exceptional cases mineral impurities in the individual coke particles may constitute up to about 30% but normally they will only constitute from 5% or less up to about 20%.
Despite the irregular shape the coke particles do not cause nearly so much wear on the parts of the briquette presses with which they come into contact as the mineral particles sorted out by the flotation, the hardness of which is considerably higher than that of the coke particles.
Notwithstanding the intimations in the abovementioned international publication WO No 83/04049 that the binding effect mostly resides with the mineral components of the fly ash, the fly ash coke exhibit satisfactory binding effect even when only employed in small amounts in the briquettes. The binding effect possibly is connected with the fact that by microscopical investigations it has been found that a small amount of graphite is condensed on the surface of the coke particles.